Pore free electrode formed of conductive titanium suboxide particles and hardened thermoset resin

ABSTRACT

An electrode for a bipolar cell or battery comprises a plate like body made of hardened resin containing particles of titanium suboxide or other electrically conductive particulate arranged to form electrical paths. A method of testing the body for porosity is also disclosed.

The invention relates to electrodes for use in a battery, typically abipolar lead-acid battery.

It is known to make bipolar plate electrodes for this purpose from leadand lead alloys. Ideally the electrodes are very thin to reduce the sizeand weight of the battery but thin sheets of lead metal and lead alloysare difficult to seal around the edges. A reliable seal is required inbipolar batteries to prevent conductive paths of electrolyte beingformed from one side of the bipolar plate to the other, which wouldcause self discharge of the battery. The plate electrodes are notentirely resistant to galvanic corrosion which generally results inthrough-plate porosity in the form of pinholes (and the electrodes areheavy if manufactured in greater thickness to overcome this problem).Proposals to reduce the effective weight of the lead include the use ofporous ceramics with lead infiltrated into the pores (which need to beof fairly thick section to be mechanically robust, and are thereby stillrather heavy); and the use of glass fibres and flakes coated with lead,lead alloy, or doped tin oxide, or lead oxides as conductive particulatein a thermoplastic resin matrix but such electrodes are complex andexpensive to produce. Carbon based materials have been tried, but mostforms are susceptible to electrochemical oxidation.

Plates made exclusively of the Magneli phase suboxides of titanium (ofthe general formula Ti_(n)O_(2n−1) (where n is an integer greater than 4or greater) satisfy many of the criteria above. However, they areexpensive to make, are brittle, and do not easily accept surfacefeatures, for example to accept and retain the battery paste coating.

This invention is based on the realisation that if the plates can bemade from the Magneli titanium suboxide material in particulate form ina suitable polymeric matrix, most, if not all, of these weaknesses canbe overcome.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a DC power supply attached to a test electrode with twomonopoles.

According to the invention in one aspect there is provided an electrodecomprising a shaped substantially pore-free body of hardened resin, thebody having electrical paths defined by contacting particles of titaniumsuboxide of the formula Ti_(n)O_(2n−1) where n is 4 or greater.

The particulate titanium suboxide is preferably selected to provide ahigh level of conductivity; Ti₄O₇ and Ti₅O₉ are preferred. Somesuboxides have low conductivity and poor corrosion resistance andpreferably are avoided; an example is Ti₃O₅. Although the particles canbe provided as a mixture of the Magneli phases it is important that thepresence of lower oxides such as TiO, Ti₂O₃, Ti₃O₅ is minimised andpreferably entirely avoided.

It is a preferred feature of the invention that the particle sizedistribution is selected so that the particles will contact each otherintimately to create electrical paths and provide conductivity.Preferably the particle size distribution is relatively narrow sincethis gives good electrical connectivity. Preferably the particles have aparticle size distribution with a standard deviation of less than about50% of the mean particle size. Polymodal mixtures can also be used butcare must be taken to ensure that the populations of smaller particlesdo not reduce the electrical connectivity of the populations of largerparticles.

We have found that specific particle sizes and particle sizedistributions are required for making electrodes of a specific thicknessbut a mean particle size (by volume) of around 100 to 150 micrometers issuitable for an electrode of 1 to 2 mm thickness. For making thinnerelectrodes which may be preferred, smaller particles are required of theplate is to be pore free. However, if the average particle size is smallit is more difficult to achieve a suitably narrow particle sizedistribution to give a good conductivity.

The powder is manufactured by methods such as are taught in U.S. Pat.No. 5,173,215. The manufacturing conditions are adjusted to ensure thatthe powder has a high proportion of the Ti₄O₇ and Ti₅O₉ crystallography(to produce high conductivity) and effectively none of the non-MagneliTi₃O₅ material (which causes poor corrosion resistance and lowconductivity). The precursor TiO₂ powder is chosen or treated to producea Magneli phase suboxide powder with particle size distribution requiredfor good conductivity.

The resin may be selected from a wide variety of materials. Preferredare thermoset resins. One suitable resin to manufacture a corrosionresistant plate is an uncured epoxy such as Araldite® PY307-1, inconjunction with HY3203® hardener, both materials being available fromVantico Ltd. This has been found to be particularly resistant to anodiccorrosion and to make a pore free plate, although other resin systemswill produce satisfactory products. Thermoset resins are particularlysuitable for the manufacturing of good conductivity plates since theyare handled in a hot press, which also presses the particles togetherfor intimate electronic contact, and they also shrink somewhat oncuring, further pushing the particles together. Other candidatethermoset resins include epoxyphenols, novolac resins, bisphenol A basedepoxy resins, bisphenol F epoxy resins; polyesters (saturated,unsaturated, isophthalic, orthophthalic, neopentylglycol modified,modified vinylester; vinylester urethane and the like. Some grades ofthese polymers have been found to exhibit a relatively excessive amountof shrinkage on curing coupled with a relatively poor adhesion to theparticles which allows interconnecting voids to appear around thesurfaces of the particles which makes them unsuitable for producingsubstantially pore-free plates. However, low shrink and other additivesmay be included in commercial grades of these resins, provided that theydo not have a detrimental effect on the chemical stability of the resinin the acid electrolyte. Some polymers have been shown to be unstable inthe polarised presence of an acid electrolyte. Some commercial resinshave a mould release agent preblended in the mixture and these should beavoided in this application since they can adversely affect the adhesionof the active battery materials and potentially affect the corrosionstability of the plate and also the surface chemistry (surface tensionetc.) of the battery acid electrolyte. The chosen resin will preferablybe one which is resistant to the electrolyte acid, especially where theelectrode is for bipolar batteries.

U.S. Pat. No. 5,017,446, discloses the inclusion of a wide range ofconductive fillers in a thermoplastics resin. We have found that thehigh volume fraction of particles disclosed in U.S. Pat. No. 5,017,446means that the finished electrode is very porous and unsuitable for useas a bipolar electrode unless great care is taken in ensuring that theparticle size distribution of the particles is such as to engender avery close packing density, such as a bimodal or trimodal distribution.In addition, the matrix of 60% volume solids in a thermoplastic, whichthis source uses as an example has very poor flow properties even at thehigh melt temperatures (370° C.) cited, and would be unsuitable forinjection moulding—which is the preferred mass production technique forthermoplastic materials. In order to improve both the porosity and theflow characteristics of the melt, it is necessary to significantlyreduce the fraction of solid particles in the mixture to less than about35% vol. It is clear from Table III of U.S. Pat. No. 5,017,446 that theresulting material would have a resistivity which would be unsuitablefor use in a bipolar lead-acid battery where the threshold value ofsuitable resistivity is generally accepted to be lower than 1 Ohm.cm. Inexample 6, U.S. Pat. No. 5,017,446 indicates that a resistivity of 9.2Ohm.cm was achieved which is unsuitable for use as a bipolar electrodein a lead-acid battery. The present invention is of a material which hassuitable resistivity and porosity, and can be made without the need forvery careful particle size management and allows a well known industrialprocess to manufacture.

The conductivity of the titanium suboxide particles may be improved bycontact with a gas such as helium or hydrogen for a period, say up to 24hours before being incorporated in the resin composition in manufactureof the electrode.

The relative proportions of resin and suboxide powder and the particlesize distribution of the suboxide powder will affect the properties ofthe electrode. For example an electrode will tend to have lowconductivity if:

-   -   too high a volume proportion of resin is used; and/or    -   the plate or other body shape is pressed in manufacture with too        little or with uneven force; and/or    -   the particle size distribution leads to low packing density;        and/or    -   the average particle size is too small; and/or    -   the resin shrinks insufficiently on curing; and/or    -   any excess resin is not ejected from the mould as flash due to        either the resin curing too quickly, the viscosity of the resin        being too high (either intrinsically or by virtue of the mould        temperature being too low), or by the mould clearances being too        small.

The electrode will tend to have unacceptable through porosity if:

-   -   too low a volume proportion of resin is used; and/or    -   the particle size distribution provides a low packing density        such that there is more volume of inter-particle voids which        needs to be filled with resin and so the effective volume        proportion of resin becomes low and/or    -   the average particle size is too large; and/or    -   the resin shrinks excessively in manufacture of the electrode        and by virtue of poor adhesion to the particles forms cavities        adjacent to and around the particles on curing; and/or    -   the resin cures too slowly, is of low viscosity (either        intrinsically or by virtue of the mould temperature) or the        mould clearances are too large that significant amounts of resin        are lost from the mould.

When manufacturing the body it is preferred to have a slight excess of athermoset resin. In press moulding the conducting particles are pressedtogether to form low resistance conductive paths. Any excess resin isejected from the mould as “flash” before the final cure of the material,which occurs in the press, under pressure, thus locking in theelectrical connectivity.

Particles with high (rods, fibres) or low (flakes) aspect ratio of thetitanium suboxide can also be present to increase connectivity betweenthe electrically conductive suboxide particles in the electrode. Highaspect particles are especially favoured because they provide longerunbroken electrical paths, so increasing conductivity.

A preferred electrode of the invention is a plate which has thefollowing combination of features:

-   -   is electronically conductive, i.e. an overall electrical        conductivity greater than 0.5 S.cm⁻¹ more specifically has an        orthogonal conductivity of at least about 1 S.cm⁻¹ which is        relatively uniform across the face of the plate;    -   has essentially no through porosity (which would allow ionic        species to travel through the pores causing self discharge of        the battery) as demonstrated by a leakage current of less than 1        A/m²;    -   is resistant to chemical attack by the materials in a lead-acid        battery (this is primarily the acid, but also the oxidant PbO₂        and the reductant Pb metal);    -   is resistant to galvanic corrosion (especially at the oxidation        potential which occurs during recharge of the positive side of        the bipolar plate);    -   provides an intimate and adherent surface to the active        chemicals in the battery (such as PbO₂, PbSO₄, Pb, tri-basic        lead sulphate, tetra-basic lead sulphate);    -   is mechanically robust in thin sections. Whilst the cured resin        particulate electrode is generally sufficiently robust, the        presence of a moulded-in grid on the surface of an otherwise        flat plate increases the stiffness of the thin plate;    -   does not catalyse the production of oxygen or hydrogen at the        potentials which occur during the recharge of the battery;    -   provides a surface to which adhesives and sealants and/or        mechanical seals can be applied;    -   ideally has some surface features, (such as a triangular,        square, hexagonal or other tessellated pattern grid) which will        allow the active paste material to be easily and uniformly        spread onto the cells thus formed, and to restrict the movement        of the paste during the charge and discharge cycling of the        battery, and    -   ideally is of low weight.

In another aspect the invention provides a method of making anelectrode, the method comprising mixing an unhardened resin, a hardenertherefor, and the particles of the Magneli titanium suboxide and pouringthe mix into a mould therefor to form the shaped body.

In one preferred method the resin and hardener are heated, the particlesof titanium suboxide are added to form a dough, which is then added to apreheated mould. In another preferred method the resin components andthe suboxide particles are first formed into a sheet moulding compoundwhich can be placed uniformly in the mould because it can be handledeasily.

The method preferably includes the step of placing the mould in a heatedpress and applying pressure. The pressure may be about 2000 Pa and thetemperature at least 35° C., preferably at least 70° C. In oneembodiment the method includes the further step of removing the shapedarticle from the mould and cleaning the surfaces by processes such asgrit blasting, applying corona discharge and plasmas, and other surfacecleaning techniques.

The method further includes the step of applying a battery paste to theelectrode. Different amounts of paste may be applied to different areasof the electrode.

Preferably the method includes the step of first applying a thin layerof metal to the electrode before the paste is applied. In one preferredtechnique the method includes applying the metal layer by electroplatingand adding dispersoids to the plating solution.

In another preferred feature the method includes the step of pressing athin foil, say up to about 200 micron thick, of metal on to the surfaceof the electrode whilst in the moulding press and the resin is curing.Other methods include plasma or flame spraying, sputtering, chemicalVapour deposition and the like.

Low viscosity resins are preferred to wet the external surface of theparticles which will enhance low porosity say less than about 50 Pa.s at20° C. These resins will also tend to infiltrate into the microscopicsurface features of the particles to improve mechanical strength. Theviscosity may be lowered by pre-heating or by selection of suitableresins. However extremely low viscosity resins should be avoided for thereasons stated above.

Coupling agents such as silanes to contact the surface of the particlesmay be used to improve the adhesion and wetting of the resin to thesuboxide particles to enhance low porosity and high mechanical strength.The coupling and/or wetting agents (such as silanes and othersurfactants) can be advantageously used on plates which do not have themetallic layer imposed. The pasting of the plates is carried out in theusual way, with conventional leady oxide paste or other lead containingpastes. The existence of the impressed surface features means that acontrolled volume of paste is applied to the grid area of the plates;pasting with thicker or thinner layers can be managed by having the gridhigher or lower. It is also possible, by adjusting the shape of themould to have some areas with thick paste and other with thin paste inorder to optimise the discharge characteristics of the battery. Thepaste on the electrode can be cured in the usual way.

In another aspect the invention provides a battery including anelectrode as defined herein or when made by a method as defined herein.

Preferably the battery comprises a plurality of electrodes and an acidelectrolyte.

With pasted and cured plates, a battery is assembled using a number ofbipolar plates, appropriately oriented, and a single positive monopoleat one end and a single negative monopole at the other. Absorptive glassmats can be advantageously inserted between each plate. Sealing of theplates is achieved in the laboratory by the use of gaskets ofappropriate thickness and made of say butyl or silicone rubber sheet.The entire assembly is held together by metal straps and bolts ofsuitable length. In a commercial battery, in a preferred feature of theinvention, the plates are sealed into a pre-moulded plastic container,with slots for each plate. A certain amount of compression of the glassmat and of the paste can be engendered by correct dimensioning of thecontainer. Such compression has been found to aid the adhesion of thepaste to the bipolar electrode substrate. Low concentration sulphuricacid can be added followed by a lid having grooves which will seal ontothe edges of each plate, placed on the top. The lid can advantageouslyalso contain a suitable gas pressure regulating system.

The battery is then electrically formed in the usual way. As theformation takes place, then the acid increases in strength, by theconversion of the sulphate-containing paste to PbO₂ on the positiveplate and Pb metal on the negative. The initial strength of thesulphuric acid should be chosen to ensure that the final strength of theacid is in the range 30-40% by mass of sulphuric acid, or even higher.

Phosphoric acid can also be advantageously added in part or totalreplacement of the more usual sulphuric acid.

Batteries made by this method have high power and energy density (W/m³,Wh/m³), high specific power and energy (W/kg, Wh/kg.) They have highcycle life, even in deep discharge conditions, and can be manufacturedcheaply with conventional technology.

In a bipolar battery it is important for efficient discharge at highrates that the monopolar or end electrodes have excellent planarconductivity. By this invention monopolar plates can be made bysubstituting for one side of the mould a flat plate and then placing ametallic grid or mesh in the mould before the uncured resin and thesuboxide materials are placed in the mould. When the mould is closed andthe resin is cured, the metal grid or mesh will be pressed into one sideof the formed electrode, giving it excellent planar conductivity for thepurposes of a monopolar or end plate. Of course, the metal grid or meshshould not be exposed to the electrolyte otherwise it will corrode.Preferably metal studs are electrically attached to the metal grid ormesh to provide terminal connections. Lead or lead alloy foils can alsobe advantageously applied to the reverse face of the electrode in themould instead of the metal grid or mesh to provide good planarconductivity for the monopolar or end electrodes.

Metal plates, grids or meshes may be advantageously incorporated intothe bipolar plates in order to increase the planar conductivity andensure good current distribution over the full area of the electrodes.Cooling channels can be introduced into the bipolar plates in likemanner.

In another aspect the invention includes a method of testing to confirmthe absence of invisible micropores which lead to though porosity in anelectrode before pasting, comprising placing the electrode in asimulated battery and measuring the flow of current over time.

A satisfactory electrode will have a current leakage of less than 1 A/m²over 28 days when tested in the apparatus of Example 2.

In order that the invention may be well understood it will now bedescribed with references to the following Examples.

EXAMPLE 1

24 g of ARALDITE PY307+1 resin and 8.8 g of the HY3203 hardener wereweighed out into separate containers and pre-warmed in an oven at 50° C.for a minimum of 7 minutes. These materials are available from VanticoLtd. They were then thoroughly mixed together and 65 g of the Magnelisuboxide powder as below is added and mixed in thoroughly to form adough. The phase analysis of the Magneli suboxide powder was measured byX-ray diffraction as:

Ti₄O₇ 26% Ti₅O₉ 69% Ti₆O11 5%

The particle size distribution was measured on a Malvern Mastersizer tobe:

-   100 vol % below 300 micrometers-   95 vol % below 150 micrometers-   90 vol % below 125 micrometers-   50 vol % below 85 micrometers-   10 vol % below 40 micrometers

The dough was evenly spread into a mould that has been pre-warmed to 75°C. Even spreading is important to achieve uniform conductivity acrossthe face of the plate. The laboratory mould is of a “window frame” typeand consists of two platens and a frame. The mould cavity has an area of149×109 mm (0.01624 m²) and will therefore produce plates of this size.The volume of dough was sufficient to produce a plate about 1.5 mm thickat the base of the grid cells. Two locating pins at diagonal corners areused to locate the various parts of the mould. Spacer levers areavailable to re-open the mould to eject the manufactured part aftermoulding is completed. Both platens can be fitted with plates which havemachined slots 1 mm deep in the face, so that the moulded part can havea raised grid on either surface. In the example, this grid covers thecentral 136×96 mm. The grid of the laboratory plates did not extend tothe perimeter of the plate to provide a flange for sealing. Thedimensions of the grid can be changed by altering the shape of themould, and thus different volumes of active paste material will beapplied to the plates in a controlled manner.

The mould can be advantageously treated with an appropriate mouldrelease agent such as Frekote 770NC®. The mould was closed and placed ina heated press at 75° C. The mould was initially pressed at 70 kN (1137Pa) for 5 seconds and then 100 kN (1625 Pa) for 25 minutes. The mould isopened and the resulting plate is extracted. Any flashing is removedwith a metal spatula.

The conductivity of the plate was then tested and was found to be in therange 1-2 S.cm⁻¹. In this example, the density of the final plate wasaround 2.2 g/cc. Higher pressing pressures produce higher levels ofconductivity. Thus the preferred range of densities for the finalproduct is in the range of 1.8 to 2.4 g/cc or above

The surface of the plate was cleaned by grit blasting, in a blastchamber such as a Gyson Formula F1200®. The blast gun was supplied withair at a pressure of 0.8 MPa. Alumina was used for the blast medium,although other blast conditions and other cleaning methods willundoubtedly produce satisfactory results. The blasting was carried outmanually until the entire surface was uniformly matt grey in colour.Tests with surface impedance scanning techniques have shown that thisblasting in this fashion produces a plate with very uniform surfaceimpedance. The surface of the plate may also be further modified bytechniques such as corona discharge or by the application of plasmas.

The plates were pasted with active material and assembled into batteriesas below. They satisfy all the criteria above. Better results wereobtained if a thin metallic layer is first applied to the grid area ofthe plates. This layer can be of pure lead, or of lead alloys (with, forinstance, antimony, barium, bismuth, calcium, silver, tin, tellurium)and be applied in a number of ways such as electroplating, sputtering,thermal evaporation and deposition, chemical vapour deposition, lead andlead alloy shot blasting, plasma or thermal spraying or by directapplication of thin metal foils in the pressing mould. It is anadvantage of the invention that a wider variety of alloys can beconsidered than has previously been available to the lead-acid batteryengineer, where the alloys must not only satisfy corrosion conditions,but also strength criteria and an ability to be fabricated into metallicgrids. One convenient way of applying the interlayer in the laboratoryis by electroplating as follows:

One side of the flanges were painted with a stopping-off lacquer such asLacomit® from HS Walsh & Sons Ltd. The plate was then sealed with arubber O-ring onto the bottom of a plastic plating tank with thestopped-off flange uppermost. A lead metal strip was pressed against theother side of the flange to provide an electrical connection. Whenplating the side which will be used as a positive, about 500 ml of aplating solution such as 27% lead/tin methane-sulphonic acid, containinga starter additive such as Circamac HS ST6703 (both materials aresupplied by MacDermid Canning Ltd.) was poured into the plating tank. Alarge pure lead anode was used as the counter electrode. On the platesof the laboratory size, a current of 0.5 A is applied for 7 hours, whichdeposited approximately 10 g of an alloy whose composition isapproximately 6:94 tin:lead.

Plating the negative side was similar except the plating solution islead methane-sulphonic acid (Circamac HS ST6703). A current of 0.5 A wasapplied for approximately 3 hours which deposits about 5 g of leadmetal.

Other plating solutions such as those based of fluoroboric acid can beused. The plating process can also involve the use inter alia of“dispersoids” such as titania, to produce a rougher surface finish forbetter keying with the paste subsequently applied.

Adjustments to the plating current and other additives can alsoadvantageously affect the surface morphology of the layer.

After electroplating, the plates are removed from the plating bath andwashed thoroughly in deionised water. The stopping-off lacquer isremoved with acetone.

Another convenient way is by direct application of thin metallic foilsin the pressing mould. For instance, a foil of lead with two percent tinalloy, 50 micron thick, is placed in the bottom of the preheated mouldand the resin and the powder mixture spread thereon. A second foil isplaced over the spread material before the mould is closed and the resinis cured as above. At this stage, the metallic layer, whether applied byelectroplate, direct foil pressing, plasma or flame spraying.sputtering, chemical vapour deposition, or any other method can beactivated by washing it in concentrated sulphuric acid immediately priorto pasting.

In another embodiment of the invention, a lead dioxide layer or a tindioxide (suitably doped with for instance antimony to increase theconductivity) can be applied on to the substrate by methods such asanodic electroplating, sputtering, chemical vapour deposition and likeprocesses, either directly or after the metallic layer is applied. Sucha layer is preferably applied on the positive side of a bipolarelectrode.

It is well understood in the lead-acid battery industry that a certainlow level of corrosion of a lead or lead alloy electrode improves theadhesion of the active paste (particularly the positive paste) to theelectrode. However, in the case of an interlayer of the presentinvention, if the corrosion rate is too high, the interlayer can becompletely consumed, especially under deep discharge or high overchargeconditions of a lead-acid battery. One aspect of the invention is toprovide an interlayer with different areas, some of which are highlycorrodible (which give good paste adhesion) and other areas are morecorrosion resistant (which gives long life).

The method described above produces plates which are nominally flat.However, plates with simple and compound curvature and differentperimeter shapes can be made by appropriate modification of the shape ofthe mould. When assembled into batteries, such plates will engender anappropriate shape on the finished battery to enable it to be installedmore conveniently in (for instance) a body panel of a vehicle.

EXAMPLE 2

Plate electrodes of the invention were tested before the application ofany metallic layer or active battery paste to confirm the absence of anyinvisible micropores through the plate which would allow ionic species(such as H⁺, OH⁻SO₄ ²⁻) to migrate through the plate. A suitable testingcell which simulates very closely the processes which occur in a batteryis shown in the accompanying FIG. 1. The plate was assembled as if it isa bipole in a 4V cell which also contains a fully pasted, cured andcharged positive monopole and a similar negative monopole. These arepreferably of the conventional lead grid type. 30% sulphuric acid wasplaced between the plate and the monopoles in the conventional manner. Apotentiostat was applied across the monopoles to hold the voltage acrossthe test plate (measured by two identical reference electrodes in theacid either side of the test plate) to be 2.6V—which is chosen as themaximum that will be applied across a lead acid battery bipole in normaloperation. The current flowing is noted.

We have found that a typical current observed initially to be about 0.3A/m². This holds very constant over long periods (months) when the plateis manufactured as above with the preferred epoxy resin. With otherresins, it is possible that although the current measured starts offlow, it rises over a few days or weeks by several orders of magnitude.This implies that some resins are being corroded or otherwise degradedby the acid at high oxidation and reduction potentials and that ionicporosity is being created. Such a plate formulation is unsuitable forbipolar battery electrodes and means that by using the test outlined,the person skilled in the art will be able to determine which resins arebest used in this invention.

The invention is not limited to the examples. The plate electrode mayhave a flange moulded of resin which is free of the suboxide powder.This will reduce the cost of the plate but still provide effectivesealing. The invention is applicable to electrochemical cells ingeneral, including bipolar lead acid batteries, to other types ofbatteries and to fuel cells, redox energy storage cells and the like.

This invention is not restricted to conductive particles such as thetitanium suboxides although these are known to be very highly corrosionresistant, when manufactured according to the teachings of U.S. Pat. No.5,173,215 which is required for lead-acid battery electrodeapplications. Other conductive particles can also be used such asniobium doped titanium oxides, tungsten oxides, niobium oxides, vanadiumoxides, molybdenum oxides and other transition metal oxides in bothstoichiometric and non stoichiometric forms. It is an advantage of theinvention that good conductivity electrodes can be made from relativelylow conductivity particulate materials, or by a smaller proportion ofrelatively expensive particulate materials.

1. An electrode comprising a shaped body which is formed of hardenedthermoset resin, the body having electrical paths defined by contactingconductive particles wherein i) the conductive particles are titaniumsuboxides of the formula Ti_(n)O_(2n−1) where n is 4 or greater, ii) theparticles have a size distribution with a standard deviation of lessthan about 50% of the mean particle size, iii) the body has a density of1.8 g/cc or above, and iv) the electrode is sufficiently pore-free suchthat the electrode has a current leakage of less than 1 A/m².
 2. Anelectrode according to claim 1, wherein the titanium suboxides aresuboxides selected from the group consisting of Ti₄O₇, Ti₅O₉ and Ti₆O₁₁.3. An electrode according to claim 1, wherein the particles have a meanparticle size in a range of about 50 to about 300 micron.
 4. Anelectrode according to claim 1, wherein there is a bimodal distributionof substantially uniform large particles and a proportion of smallerparticles dimensioned to fit in interstices formed between the largeparticles.
 5. An electrode according to claim 1, wherein there is apolymodal distribution of a range of particle sizes ranging from largeparticles to successively smaller particles dimensioned to fit ininterstices formed between larger particles.
 6. An electrode accordingto claim 1, including high aspect and/or low aspect particles of thetitanium suboxide to increase connectivity.
 7. An electrode according toclaim 1, having an overall electrical conductivity greater than 0.5S.cm⁻¹.
 8. An electrode according to claim 7, having an orthogonalconductivity greater than
 1. S.cm⁻¹.
 9. An electrode according to claim1, in the form of a plate which is flat or has curvature.
 10. Anelectrode according to claim 1, which has a metallic layer applied to asurface thereof.
 11. An electrode according to claim 10, where themetallic layer has areas of differing corrosion rates.
 12. An electrodeaccording to claim 1, having a lead dioxide or doped tin dioxide layerapplied to a surface thereof.
 13. An electrode according to claim 9,wherein the plate has a flange adapted to secure the electrode to acasing of a cell.
 14. An electrode according to claim 13, wherein theelectrode is sealed in a casing which is secured to the flange byadhesive or welding.
 15. An electrode according to claim 13, wherein theflange is free of the particles of titanium suboxide.
 16. An electrodeaccording to claim 9, wherein the plate is received in a slot in acasing therefor.
 17. An electrode according to claim 1, comprising aplate having a metal grid or mesh or sheet in the body thereof.
 18. Anelectrode according to claim 1, comprising a plate having coolingchannels in the body thereof.
 19. An electrode according to claim 1,wherein a surface of the electrode has surface deformations forreceiving and retaining active paste material.
 20. An electrodeaccording to claim 19, wherein the deformations are moulded in thesurface.
 21. An electrode according to claim 19, wherein thedeformations are formed in the surface after the electrode has beenmoulded.
 22. An electrode according to claim 19, wherein thedeformations are dimensioned and shaped to receive different thicknessesof paste in different areas.
 23. A method of making an electrodeaccording to claim 1, the method comprising: mixing the conductiveparticles, an unhardened thermosettable resin and a hardener therefor toform a mix, pouring the mix into a mould therefor and moulding the mixto form a shaped body and thereby provide the electrode.
 24. A methodaccording to claim 23, wherein the mix includes a bimodal distributionof substantially uniform large particles and a proportion of smallerparticles dimensioned to fit in interstices formed between the largeparticles.
 25. A method according to claim 23, wherein the mix includesa polymodal distribution of a range of particle sizes ranging from largeparticles to successively smaller particles dimensioned to fit ininterstices formed between larger particles.
 26. A method according toclaim 23, wherein the titanium suboxide comprises Ti₄O₇ and/or Ti₅O₉.27. A method according to claim 23, wherein the particles of thetitanium suboxide are first contacted with a gas for a period to extendthe conductivity thereof.
 28. A method according to claim 27, whereinthe gas is helium or hydrogen.
 29. A method according to claim 23,wherein the resin has a viscosity of less than about 50 Pa.s at 25° C.30. A method according to claim 23, wherein the resin, hardener andparticles are first formed into sheet moulding compound which is addedto the mould.
 31. A method according to claim 30, including the step ofapplying foils of metal to one or both surfaces of the sheet mouldingcompound.
 32. A method according to claim 23, including the step ofremoving the shaped body from the mould and cleaning its surfaces.
 33. Amethod according to claim 32, wherein the method of cleaning includesgrit blasting.
 34. A method according to claim 23, wherein excess resinis ejected from the mould during pressing.
 35. A method according toclaim 23, including the step of applying a thin layer of metal to theelectrode before a paste is applied.
 36. A method according to claim 23,including the step of pressing a metal foil on to a surface of theelectrode whilst in the moulding press while the resin is curing.
 37. Amethod according to claim 36, wherein the metal foil is up to 200 micronthick.
 38. A method according to claim 35, including applying the metallayer by electroplating using a plating solution and optionally addingdispersoids to the plating solution.
 39. A method according to claim 35,including treating a surface of the metal layer with a corona dischargeor plasma.
 40. A method according to claim 35, including adding acoupling and/or wetting agent to the paste.
 41. A battery including anelectrode according to claim
 1. 42. A battery according to claim 41,comprising a plurality of electrodes and an acid electrolyte.
 43. Anelectrode according to claim 1 wherein the hardened thermoset resincomprises an epoxy.
 44. An electrode according to claim 1 having aconductivity in the range of 1 to 2 Scm⁻¹.
 45. An electrode according toclaim 1, wherein the conductive particles comprise particles of Ti₄O₇,Ti₅O₉ and Ti₆O₁₁.
 46. A substantially pore-free electrode comprising ashaped body which is formed of hardened thermoset resin, the body havingelectrical paths defined by contacting conductive particles, wherein i)the conductive particles comprise suboxide particles of Ti₄O₇, Ti₅O₉ andTi₆O₁₁, ii) the particles have a size distribution with a standarddeviation of less than about 50% of a mean particle size, and iii) thebody has a density of 1.8 g/cc or above.
 47. An electrode according toclaim 46, wherein the suboxide particles consist essentially of Ti₄O₇,Ti₅O₉ and Ti₆O₁₁.
 48. A bipolar lead acid battery comprising a plateelectrode comprising a shaped body which is formed of hardened thermosetresin, the body having electrical paths defined by contacting conductiveparticles, wherein i) the conductive particles comprise suboxideparticles of Ti₄O₇, Ti₅O₉ and Ti₆O₁₁, ii) the particles have a sizedistribution with a standard deviation of less than about 50% of a meanparticle size, iii) the body has a density of 1.8 g/cc or above, and iv)the electrode is sufficiently pore-free such that the electrode has acurrent leakage of less than 1 A/m².